That's not a question we have to debate until it happens.
But seriously ... part of the whole story of man's history and salvation includes the fall of man through sin, the fall of the created order because of our position as priests over creation, Christ's entry into human flesh and redemption of mankind, and so on.
Aliens aren't part of that. Did they fall? Did Christ redeem them in some way? That's His business. If they even exist.
Asking such questions reminds me of how many angels can dance on the head of a pin, or whether God can create a rock too big for Himself to lift.
Ask your priest. But it's more important to look at why such questions concern you - since you are asking - than the actual answer, since it will probably never come up.
Only the theological points behind it are worth understanding, and we don't need to invoke outer-space aliens to understand them.
God be with you.